The invention relates to the stabilization of blood-staining solutions, especially those which contain thiazine dyes.
Differential blood counts have been performed for a long time by means of the known staining solutions according to Giemsa, May-Grunwald, Leishman and Wright. An important disadvantage of these staining solutions is that the investigation results from various laboratories cannot be compared with one another because of the different quality of the commercially available dyes (J. Clin. Path. 28, 680 (1975)). There are not only large differences in quality between the dyes from various manufacturers, but the dyes are also subject to chemical changes over time. Because of the instability of, in particular, the methylene blue molecule which especially in an alkaline medium, is converted by oxidative demethylation into the next-lower homologues azure A, azure B, azure C and thionin, the said staining solutions cannot be produced with constant quality and can therefore also not be standardized. Essentially, the staining properties change due to a decrease in the optical density of the thiazine components, which is determined at about 645 nm. As a result, there is a continuous shift in the blue/red color ratio and, after some time, this leads to stainings which are no longer acceptable. Moreover, preservation of the reproducible staining properties is made more difficult by the fact that the dyes in the staining solutions tend, especially at low temperatures, to crystallize out of solution, with eosin crystallizing out at a higher rate than the thiazine components. Standardization of the staining reactions, which is indispensable in this field because of increasing automation, is virtually impossible with the known blood-staining solutions.
It is already known from German Offenlegungsschrift 3,533,515 that the stability of staining solutions can be greatly increased by an addition of dimethylammonium sulfate. Whereas unstabilized solutions, depending on the quality of the raw materials employed, tend to suffer precipitations of dye crystals sometimes even at room temperature, but in most cases at lower temperatures, the staining solutions stabilized with dimethylammonium sulfate remain clear, for example at room temperature, but a formation of precipitates occurs in some cases in the temperature region around 9.degree. C., with a resulting deterioration in the staining quality.